1. Field of the Invention
This invention relates to a drive method and drive apparatus for capacitive display apparatuses such as EL (electroluminescent) display apparatus.
2. Description of the Prior Art
A thin form EL element of double insulation type (or 3-layer structure), for example, is constituted as follows.
As shown in FIG. 1, transparent electrodes 2 made of In.sub.2 O.sub.3 strips are formed in parallel on a glass substrate 1. Over these electrodes 2, a dielectric material layer 3a such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 and Al.sub.2 O.sub.3, for example; an EL layer 4 made of ZnS which is doped with activating agent such as MN; and a dielectric material layer 3b such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.2 and Al.sub.2 O.sub.3 similar to a dielectric material layer 3a are formed to thickness from 500 to 10000 .ANG. each, to make a 3-layer structure by means of thin film forming technology such as evaporation deposition or sputtering. On this 3-layer structure, back electrodes 5 made of parallel aluminum strips are formed in the direction perpendicular to the above-mentioned transparent electrodes 2.
Because the thin film EL element is made of a structure where the EL material 4, sandwiched by the dielectric material layers 3a and 3b, is interposed between the electrodes 2 and 5, it can be regarded as a capacitive element in terms of an equivalent circuit. Also as will be clear from the brightness-voltage curve shown in FIG. 2, the thin film EL element is driven by applying a relatively high voltage of about 200 V. The thin film El element has features of emitting light of a high brightness with an alternate electric field, and lasting for a long service life.
In a display apparatus which uses the thin film EL element as the display element, one of the transparent electrode 2 or the back electrode 5 is used as a data electrode, and the other is used as the scanning electrode. The data electrode is fed with a modulation voltage in accordance to the data to be displayed, and the scanning electrode is line-sequentially fed with writing voltage. A method (so-called symmetrical drive method) for driving the EL element, where a scanning cycle of one display frame completes with a 1st field scanning period when the writing voltage has a polarity with respect to the data electrode, and a 2nd field scanning period when it has another polarity, is generally adopted to drive a thin film EL element which is of alternate voltage-driven type and is capable of maintaining a good display quality.
With this drive method, writing voltage and modulation voltage superimpose to enhance or cancel each other at each pixel, corresponding to an intersection of data and scanning electrode among the EL layer 4, resulting in the effective drive voltage applied to a pixel being above or below the light emission threshold voltage. Thus, each pixel emits or does not emit, to thereby provide the specified display.
When creating a gradation display by changing the brightness of each pixel in multiple steps with such a display apparatus as described above, a pulse width modulation method, which changes the pulse duration of the modulation voltage applied to the data electrode in accordance with the gradation data to be displayed; an amplitude modulation method where the amplitude of the modulation voltage is changed in accordance with the gradation data to be displayed; or the like have been thus far adopted.
FIG. 3 is a block diagram explanatory of a prior art thin film EL display apparatus using the above-mentioned pulse width modulation method.
In FIG. 3, the display section 21 consists of, for example, a thin film EL element shown in FIG. 1. Scanning electrodes Y1, Y2, . . . , Ym-1 Ym of the display section 21 are connected to a scanning electrode drive circuit 22. Data electrodes X1, X2, . . . , Xn-1, Xn of the display section 21 are connected to data electrode drive circuit 23. The scanning electrode drive circuit 22 and the data electrode drive circuit 23 are connected to a display control circuit 24 which controls these circuits.
The data electrode drive circuit 23 variably sets the rising and falling timings of the modulation voltage V.sub.M which is applied to each of the data electrodes X1-Xn in accordance with the gradation display data sent from the display control circuit 24. The scanning electrode drive circuit 22 applies writing voltage -V.sub.N and Vp, which are of different polarities across adjacent scanning electrodes Y. It further differentiates the polarities of the writing voltages applied to a same scanning electrode Y in the 1st field scanning period and 2nd field scanning period.
FIGS. 4(A)-4(C) shows a voltage waveform which is applied to an arbitrary picture element during the 1st field scanning period in driving the thin film EL display apparatus. FIGS. 5(A)-5(C) shows a voltage waveform applied to an arbitrary picture element during the 2nd field scanning period. Among these, FIG. 4(A) and FIG. 5(A) show the waveforms of the modulation voltages V.sub.M which are applied to the data electrodes X. FIG. 4(B) and FIG. 5(B) show the waveforms of the writing voltage -V.sub.N, Vp which are applied to the scanning electrodes Y, respectively. FIG. 4(C) and FIG. 5(C) show the waveforms of the drive voltages which are applied to the picture elements.
In the 1st field scanning period, as shown by the solid and dashed lines of the FIG. 4(A), the greater the value of gradation display data, the faster the rise time is set for the modulation voltage V.sub.M, to obtain long pulse duration. Because the writing voltage at this point is -V.sub.N of negative polarity as shown in FIG. 4(B), in the case of the drive voltage shown in FIG. 4(C) the period of time when the light emission threshold V.sub.th is exceeded (period during which the modulation voltage is additionally superimposed onto the writing voltage) increases as the pulse duration of the modulation voltage V.sub.M increases. Thus gradation display is achieved in accordance with the pulse duration of the modulation voltage V.sub.M.
On the other hand, in the 2nd field scanning period, as shown by the solid and dashed lines of the FIG. 5(A), the larger the value of display data, the faster the fall time is set for the modulation voltage V.sub.M, to obtain short pulse duration. Because the writing voltage at this point is Vp of positive polarity as shown in FIG. 5(B), in the case of the drive voltage shown in FIG. 5(C) the period of time when the light emission threshold -V.sub.th is exceeded (period during which the modulation voltage V.sub.M and the writing voltage cancel each other) increases as the pulse duration of the modulation voltage V.sub.M decreases. Thus gradation display is achieved in accordance with the pulse duration of the modulation voltage V.sub.M.
However, in the drive method of the prior art described above, considerable electric power is consumed because the writing voltage -V.sub.N and Vp and the modulation voltage V.sub.M are charged and discharged once every time one line of the scanning electrodes Y is driven. This results in an unfavorably large power supply apparatus and poor reliability of the display apparatus.
In the case of an EL display panel of 640 * 400 picture elements, power consumption is calculated as follows. The calculation is made by next equation (1) based on the assumption that the EL display panel can be equivalently regarded as a capacitor, with the capacitance of the scanning electrode being 2200 pF per one line, threshold voltage V.sub.th of light emitting being 200 V, modulation voltage V.sub.M being 50 V and field frequency f being 60 Hz. EQU (Power consumption)=(Field frequency)*(Capacitance)*(Voltage).sup.2( 1)
Then power consumption through charging and discharging of writing voltage Vp and -V.sub.N, that is the power P.sub.W required in writing, is given as EQU P.sub.W =60(Hz)*2200(pF)*400(lines)*200.sup.2 (V).apprxeq.2.1(W)(2)
Further power consumption through charging and discharging of modulation voltage V.sub.M, that is the power P.sub.M required in modulation, is given as EQU P.sub.M =(400 lines*60 Hz)*(2200 pF*400 lines)*(50 V).sup.2 .apprxeq.52.8(W)(3)
As the above calculations show, modulation power P.sub.M accounts for a large part of the power consumption (P.sub.W +P.sub.M).